1. Field of the Invention
The present invention relates to a hot water supply system for maintaining a constant temperature of hot water, and more particularly, a hot water supply system for maintaining a constant temperature of hot water that enhances an overshoot and an undershoot of the temperature of the hot water which temporarily occur depending on variation of usage of the hot water.
2. Description of the Related Art
In general, a hot water supply system (hereinafter, referred to as “water heater”) is used for heating and supplying water at a predetermined temperature within a short time. A performance of the water heater is primarily determined depending on how rapidly a temperature of hot water reaches a user-set temperature, and whether hot water maintaining the user-set temperature can be supplied even if usage of the hot water varies.
FIG. 1 is a diagram illustrating a configuration of a known water heater having a by-pass valve 5, and FIG. 2 is a graph illustrating a change in temperature of hot water depending on a decrease in usage of hot water in the water heater. FIG. 3 is a diagram illustrating a configuration of a known hot water supply system having a mixing valve 8, and FIG. 4 is a graph illustrating a change of a temperature of hot water depending on a decrease of usage of the hot water in the system shown in FIG. 3.
The water heater shown in FIG. 1 includes a flow sensor 1 that senses a flow rate of inflow water, an inflow water temperature sensor 2 that measures a temperature of the inflow water, a heat exchanger 3 that transfers heat from a heating device (burner) to the inflow water, a by-pass pipe 4 that directly connects an inflow pipe and an outflow pipe with each other, a by-pass valve 5 for supplying the inflow water directly to the outflow pipe through the by-pass pipe 4 without passing through the heat exchanger 3, an outflow water temperature sensor 6 that measures a temperature of hot water in the outflow pipe, and a flow control valve 7 that controls an amount of the hot water flowing in the outflow pipe.
When a user, while using the hot water, decreases the usage of the hot water from a large flow rate to a small flow rate at a time P1 in the graph of FIG. 2, a controller (not shown) senses the decrease of the flow rate through the flow sensor 1 to reduce a heating power of the burner. However, as hot water of the large flow rate has already been supplied to the heat exchanger 3 before the usage of the hot water is decreased, a large amount of heat has already been supplied to the inside of the heat exchanger 3. As a result, in this state, even though the flow rate passing through the heat exchanger is decreased to thereby reduce the heating power of the burner, hot water having a temperature higher than a set temperature temporarily flows out.
In order to decrease the time period during which the hot water having the temperature higher than the set temperature flows out (hereinafter referred to as “overshoot”), the controller rapidly reduces the heating power of the burner for a predetermined time. Therefore, as shown in the graph of FIG. 2, following the period during which the hot water having the temperature higher than the user set temperature temporarily flows out (see arrow a), hot water having a temperature lower than the user set temperature flows out for a predetermined time (hereinafter referred to as “undershoot”) (see arrow b).
As such, in order to prevent an outflow temperature from being temporarily increased when the user changes the hot water to a small flow rate from a large flow rate, the by-pass valve 5 that is closed in normal times is opened, such that inflow water is sent directly to the outflow pipe without passing through the heat exchanger 3 when the overshoot occurs due to the rapid decrease in the flow rate. The low-temperature water sent to the outflow pipe through the by-pass valve 5 is mixed with the overshot high-temperature water to thereby alleviate the overshoot of the outflow temperature.
However, such a water heater having the by-pass valve 5 is disadvantageous, in that it cannot alleviate the undershoot in a case where the user changes to the large flow rate of hot water from the small flow rate of hot.
In order to solve the above-mentioned problem, as shown in FIG. 3, a water heater having a flow controllable mixing valve 8 has been developed instead of the known by-pass valve 5.
The water heater shown in FIG. 3 allows a small amount of inflow water to flow to the outflow pipe without passing through the heat exchanger 3 by slightly opening the mixing valve 8 even in normal times. Accordingly, because the low-temperature water which does not pass through the heat exchanger 3 flows into the outflow pipe and is mixed with the water passing through the heat exchanger 3, a control temperature of the heat exchanger 3 is set to a temperature higher than the user-set temperature. Therefore, the water passing through the mixing valve 8 is mixed with the water passing through the heat exchanger 3, such that a temperature of the hot water in the outflow pipe reaches the user-set temperature.
For example, when the usage of the hot water is rapidly decreased at a time P1 shown in the graph of FIG. 4, the controller increases the amount of the water supplied directly to the outflow pipe through the by-pass pipe 4 by further opening the mixing valve 8 to thereby alleviate a temporary overshoot. Meanwhile, when the undershoot occurs due to a rapid increase in the usage of the hot water, the controller decreases the amount of the water supplied directly to the outflow pipe through the by-pass pipe 4 without passing through the heat exchanger by further closing the mixing valve 8. Thus, the set temperature is achieved by relatively increasing the amount of the water that flows out from the outflow pipe through the heat exchanger 3.
Accordingly, when the mixing valve described above is used, a sudden change in the temperature of hot water due to a variation in the usage of the hot water can be prevented, and hot water having a temperature approximately the set temperature can be obtained. As shown in the graph of FIG. 4, the overshoot and the undershoot of the hot water temperature which occur due to the decrease in the usage of the hot water are considerably reduced in comparison with the case shown in the graph of FIG. 2.
As described above, although the water heater shown in FIG. 1 can enhance the temporary overshoot of the temperature of the hot water which occurs due to the decrease of the usage of the hot water, the water heater is disadvantageous, in that it cannot effectively deal with the undershoot when the usage of the hot water is increased.
In contrast, the water heater shown in FIG. 3 can deal with both temporary overshoot and undershoot of the temperature of the hot water according to the variation of the usage of the hot water. However, such a water heater has a problem, in that a temperature at an outlet of the heat exchanger is controlled to be higher than the user-set temperature by approximately 20° C. on the average in order to improve controlling the temperature of the hot water depending on the variation of the flow rate of the hot water. Therefore, efficiency of the water heater is decreased.
Further, a condensing water heater recovers energy, which is released when exhaust gas is condensed, and utilizes the energy for heating the hot water. In this case, when the outlet temperature of the heat exchanger is controlled to be high, the hot water outlet temperature is higher than a condensing temperature of the exhaust gas, which is approximately 55° C. or less, such that it is difficult for the exhaust gas to become condensed. Therefore, the energy of the condensed exhaust gas cannot be recovered, thereby decreasing the efficiency of the water heater.
Further, depending on a quality of the water in use, when the water contains lime, most of the lime is extracted at approximately 55° C. or more. In the case of the water heater shown in FIG. 3, assuming that a hot water set temperature is approximately 50° C., which is a temperature generally set by the user, the heat exchanger controls the hot water set temperature to be 70° C., higher than 50° C. by approximately 20° C., so as to control the hot water outlet temperature. When water which flows into the user's inflow pipe contains lime, the lime is extracted in the heat exchanger, such that the efficiency of the water heater is deteriorated after a long period of use, and heat transfer is decreased in the heat exchanger, such that partial boiling may occur, such that durability of the water heater is decreased.